Radiotherapy involves the application of ionizing radiation to a target within a patient (e.g. a tumour) so as to damage the unhealthy cells within the target, eventually causing cell death through one or multiple exposures. The radiation is harmful to both the unhealthy tissue within the target and the healthy tissue which surrounds it, and thus much research has been focussed on maximizing the radiation dose within the target while minimizing the dose outside the target. For example, the radiation may be collimated into a particular shape so as to conform to the shape of the target or to some other shape which is desirable for treatment. Various devices can be employed in such a collimation, but the most common is the multi-leaf collimator.
It has long been a goal for those working in the field of radiotherapy to combine simultaneous imaging and therapy of the patient. This is expected to lead to improved accuracy of treatment, in that the precise location of a target area can be more accurately determined at any particular time.
One system which has been proposed to achieve simultaneous imaging and treatment combines a radiotherapy system with a magnetic resonance imaging (MRI) system. An example of such a system is shown in WO 2005/081842. The magnetic coil of the MRI system is split into two coils separated by a gap, and the therapeutic radiation beam is delivered to the patient through the gap.
Another means of imaging a patient during therapy is through the use of portal imagers. A portal imager typically comprises a flat panel detector with an array of detecting elements. The detector is placed opposite the therapeutic radiation source and provides a transmission image of the radiation beam substantially along the beam axis (i.e. back towards the therapeutic radiation source). The imager thus provides an image of the radiation beam cross section. The portal imager can also provide imaging data of the patient's anatomical structure. Such data is inherently low contrast due to the high energy of the therapeutic radiation (therapeutic radiation typically has an energy in the MeV range, while radiation used for imaging purposes typically has an energy in the keV range), but is nonetheless useful. The conventional portal imager thus performs two functions, providing a check on the shape of the radiation beam (and thus the positions of the leaves of the multi-leaf collimator or other collimating device) as well as its placement relative to the patient.
The MRI function of the system in WO 2005/081842 achieves high-quality imaging of the patient undergoing therapy, but provides no feedback on the shape of the radiation beam. A portal imager would provide such feedback, but the integration of a conventional portal imager (i.e. a flat panel detector) within an MRI system would be challenging due to the space restrictions imposed by the narrow bore magnetic coils.